The cm_buf_t.userp field should never be NULL if the CM_BUF_DIRTY
flag is set but apparently it sometimes is. cm_BufWrite() requires
that the userp parameter be non-NULL. Otherwise, an assertion fails
and afsd_service.exe panics. If bp->userp is NULL, use cm_rootUserp.
The worst that will happen is the write will fail due to an access
denied error.